Sulfonylurea-Stimulated Glucose Transport Association With Diacylglycerollike Activation of Protein Kinase C in BC3H1 Myocytes

The extrapancreatic effects of sulfonylurea drugs include increased glucose uptake by certain peripheral tissues. To study this effect, we used BC 3 H1 myocytes, which are reported to respond to these drugs. Within 30 min, tolbutamide and glyburide increased [ 3 H]-2-deoxyglucose uptake in a dose-dependent manner. The inactive analogue carboxytolbutamide had no effect on glucose transport. Because increases in glucose transport may be mediated by activation of the diacylglycerol-protein kinase C signaling system, we examined the effects of these drugs on lipid metabolism and protein kinase C activity. Unlike insulin, tolbutamide and glyburide failed to increase [ 3 H]glycerol labeling of diacylglycerol or labeling of phospholipids by 32 P. After 30 min of treatment with tolbutamide or glyburide, however, membraneassociated and cytosolic protein kinase C activity were each increased. When cells were treated with 12- O - tetradecanoylphorbol-13-acetate (TPA) for 48 h to deplete certain isoforms of protein kinase C, glyburide, tolbutamide, and acute TPA treatment failed to increase glucose uptake, suggesting that TPA and sulfonylureas operate through activation of a common pathway. The effect of glyburide was additive to TPA in stimulating glucose uptake at low but not high TPA concentrations. As with insulin and TPA, extracellular Ca 2+ was not essential for sulfonylurea-stimulated glucose uptake. Staurosporine, a protein kinase C inhibitor, blocked glyburide-, tolbutamide-, and insulinstimulated glucose uptake. In intact cells, glyburide stimulated the phosphorylation of both 80,000- M r and 40,000- M r proteins, which are markers for protein kinase C activation. Addition of sulfonylureas directly to the protein kinase C assay system in vitro provoked dioleinlike effects, in that sensitivity of the enzyme to Ca 2+ was increased. Our findings suggest that tolbutamide and glyburide increase glucose uptake in BC 3 H1 myocytes by a postreceptor mechanism, which may involve direct activation of protein kinase C.